BIOLOGICAL FILTER FOR REMOVING VOLATILE HYDROPHOBIC COMPOUNDS FROM GAS EMISSIONS

Abstract: Biological filter for removing volatile, hydrophobic compounds, such as toxic styrene, from gas emissions. These are broken down by means of a fungus, which is immobilized on a bearer of inert material, such as perlite. Preferably active carbon is also added. The filter is manufactured by cultivating the fungus on the bearer with the aid of the compound which is to be removed. In order to be able to make a filter quickly with a homogeneous distribution of the fungus therein, it is preferable to use a spore forming fungus.

Description: Biological filter for removing volatile hydrophobic compounds from gas em.Lesions. The invention relates to a biological filter for the removal of volatile, hydrophobic compounds from gas emissions, whereby the gas emission is passed through the filter, in which those compounds are broken down with the aid of a fungus (fungi) which is (are) present on a bearer material. In industrial gas emissions such compounds often occur; they are polluting for the environment and are not permitted on the grounds of national environmental laws, or are only permitted in very limited quantities. They must therefore be removed from the gas emission and broken down. For example, the toxic styrene occurs in concentrations of up to approximately 1 gr/m 3 in gas emissions of factories which manu¬ facture (pol )styrene and reinforced plastic. A known technique for catching and breaking down such compounds is with the aid of biological filters, filled with compost. Micro-organ- isms are present on the compost particles, which achieve the desired breakdown. It turns out that such compost filters only work with a pH of between 5 and 9 and in the event of a high water capacity, correspond¬ ing with a water content of AO to 60%. Further the demands were made of the filters that the drop in pressure across the filter is slight, that the gas is distributed homogeneously across the filter, whereby there is no case of channel-forming, and that the bearer material for the micro-organisms holds water well and absorbs the volatile compounds. Problems in the use of the said compost filters are, that during the use the pH and the water capacity/the water content must be con¬ tinuously checked and kept at the right level. Furthermore, it appears that the filter is instable when used for the said hydrophobic com¬ pounds: it only works during a relatively short time - several weeks. And the mass transport of the gas to the water phase is small, so that the filter has a low volumetric elimination capacity. A solution could be to achieve the breaking down of the compounds especially with the aid of fungi which break down those compounds. For example, in the case of the breaking down of styrene, with the aid of "Exophiala jeanselmei" as described in Appl. Microbiol. Biotechnol (1993) 39:372-376. Such fungi also work at a relatively low pH, for example ρH=3, and with a relatively low water capacity. Furthermore, the active surface of the fungi mycelia j.n the gas phase is large and, as a result of the fact that a low water content is possible, the mass transport of the hydrophobic compounds can be higher. The fungi can be cultivated, for example, on compost; see, for example, the patent pu¬ blication DE 3807033. However, it appears that on the grounds of further essential qua¬ lities for such a filter (the pressure drop across the filter, the homogeneity of the distribution of the gas across the filter, the ca- pacity to hold water to a sufficient degree and the absorbtion effect) such a filter also only works sufficiently during a very limited time. The latter is solved according to the invention by choosing a bearer material which consists of particles of an inert, anorganic or an organic material, which holds water well. Especially good represen- tatives of such materials are: perlite and vermiculite. The use of perlite in order to cultivate micro-organisms is as such known from the Japanese patent publication J 611 1917 - however, not in a filter and in combination with a fungus as referred to here. With a filter according to the invention, in which perlite acted as the bearer material, the following results were achieved when re¬ moving styrene from a gas emission: the gas emission contained 800 mg styrene/m 3 . A quantity of 120 m 3 per m 3 filter per hour was passed for 70 days through the filter, which was moistened regularly and whose pH had dropped to 3 after 15 days, and this value remained during the rest of the time. The elimination ca¬ pacity amounted to 80 gr. styrene per 3 filter per hour. This is 1.5 times to 7 times higher than what is achieved with a compost filter and furthermore: the effective duration of the filter is considerably high¬ er. The adhesive qualities of the bearer material can be further im¬ proved if, according to a further aspect of the invention, active car¬ bon is added to it. The invention also contains a method for the manufacture of a biological filter according to the invention. That method is charac- terized by the fact, that the filter material is enriched with the desired fungus(i) by, after adding the bearer material, inserting an earth suspension and passing gas through the filter which contains the compound which is to be removed. For example, a styrene/air mixture was passed through a biofilter, with perlite as the b arer material on which an earth suspension had been placed, with an average flow of 45 1/hour and a concentration of 250 mg styrene per 3 . The styrene concentration in the gas was in- creased after 110 days to an average of 675 mg/m 3 . It appeared that after approximately 4 weeks it could be determined that a biological mass was forming. In order to have the filter function in an optimal manner it is advantageous to cultivate the biological mass via a gas flow, which contains the compound which is to be removed in a concen- tration which is comparable to that of the gas emission which is to be purified. The manufacture of a filter as referred to here can be accelerated and the result can be improved as far as homogeneity is concerned by working with a spore forming fungus. A homogenous 'occupation' of the filter can also be achieved by using bud-forming as occurs in fungi. The invention also contains the method for removing the compounds concerned from gas emissions, with the aid of a filter according to the invention. While the water content with the use of filters according to the invention is no longer as critical as in the case of compost filters, and even a low water content can have advantages with regard to the mass transport, nevertheless the filter will have to be regularly pro¬ vided with water during its operation. And in order to ensure that the growth conditions of the fungus(i) continue to be maintained, a mixture of, for example, vitamins, minerals and ammonium chloride in a sodium phosphate buffer with a pH of, for example, 5.7, is regularly, for example every three days, added to the filter material.

Claims: Claims 1. Biological filter for the removal of volatile, hydrophobic compounds from gas emissions, whereby the gas is passed through the filter, in which those compounds are broken down with the aid of a fungus (fungi) which is (are) situated on a bearer material, characterized in that the bearer material consists of particles of an inert anorganic or organic material, which holds water well. 2. Biological filter according to claim 1, characterized in that the bearer material is perlite. 3. Biological filter according to claim 1, characterized in that the bearer material is vermiculite. 4. Biological filter according to one of the claims 1 to 3, characterized in that the bearer material also contains active carbon. 5. Method for the manufacture of a biological filter according to one of the claims 1 to 4, characterized in that the filter with the desired fungus(i) is enriched by, after the bearer material has been placed, introducing an earth suspension into it and passing gas through the filter, which contains the compound which is to be removed. 6. Method according to claim 5, characterized in that the gas which is passed through the filter is the gas emission which is to be treated. 7. Method according to 5 or 6, characterized in that a spore forming fungus is placed in the filter. 8. Method according to claim 5 or 6 characterized in that a bud-forming fungus is placed in the filter. 9. Method for removing volatile, hydrophobic compounds from gas emissions, with the aid of a biological filter according to one of the claims 1 to 4. 10. Method according to claim 9, whereby the volatile, hydrophobic compound is styrene.

Inventors: COX HUBERTUS HENRICUS JACOBUS, DODDEMA HARMANNUS JOHANNES, COX, HUBERTUS, HENRICUS, JACOBUS, DODDEMA, HARMANNUS, JOHANNES
Applicants: COX HUBERTUS HENRICUS JACOBUS, DODDEMA HARMANNUS JOHANNES, TNO, COX, HUBERTUS, HENRICUS, JACOBUS, DODDEMA, HARMANNUS, JOHANNES, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST NATUURWETEN
Application year: 1994
Main classification: B01D53
IPC codes: B01D53, C12S5